The present invention relates to an apparatus and a method for distributing a liquid from a liquid distributor to a packing in an exchange column for heat and/or mass transfer processes. The apparatus and method have particular application in cryogenic air separation processes utilizing distillation, although the apparatus and method may be used in other heat and/or mass transfer processes that use liquid distributors and packing (e.g., random or structured packing).
The term, xe2x80x9ccolumnxe2x80x9d, as used herein, means a distillation or fractionation column or zone, i.e., a column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements.
The term xe2x80x9cpackingxe2x80x9d means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface area for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are xe2x80x9crandomxe2x80x9d and xe2x80x9cstructuredxe2x80x9d.
xe2x80x9cRandom packingxe2x80x9d means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column. xe2x80x9cStructured packingxe2x80x9d means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of expanded metal or woven wire screen stacked in layers or as spiral windings; however, other materials of construction, such as plain sheet metal, may be used.
The terms xe2x80x9corifice,xe2x80x9d xe2x80x9chole,xe2x80x9d and xe2x80x9caperturexe2x80x9d are used interchangeably herein to mean an opening through which a fluid may pass. Although circular orifices are shown in the drawings, the orifices may have other shapes, including irregular as well as regular shapes.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen).
Various packings may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases. The use of packing for distillation is standard practice and has many advantages where pressure drop is important. However, packed column performance is very dependent on creating and mantaining a balance between the downward flow of liquid and the upward flow of vapor locally in the packing. The distribution of the liquid and the vapor within the packing as influenced by the initial presentation of these fluids to the packing.
Initial presentation of liquid and vapor to the packing is usually made by means of distributors. A liquid distributor, the role of which is to irrigate the packing uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create uniform vapor flow below the packing, is located below the packing.
There are three main types of typical liquid distributorsxe2x80x94pipe, pan, and trough distributors. Each type is discussed briefly below.
Pipe distributors are comprised of an interconnecting network of closed pipes or ducts, typically comprising a central pipe or manifold and a number of arms or branches radiating from the central pipe. The arms are perforated to allow the liquid passing from the central pipe and into the arms to be dripped or sprayed onto a packed bed below the pipe distributor. Upwardly flowing vapor passes easily in-between each arm. Pipe distributors receive liquid from a separate liquid collector or an external source piped through the wall of the column. While simple and inexpensive to construct, pipe distributors may distribute liquid poorly when vapor gets trapped in the arms.
Pan distributors are comprised of a pan or pot having holes in the bottom for feeding liquid to the packing below and tubes or risers for the vapor to pass upwardly through the distributor. Pan distributors often make a complete seal with the wall of a column. Thus, pan distributors can act as liquid collectors as well as distributors. However, since large pan distributors are costly to build, pan distributors usually are used in smaller columns, i.e., columns with diameters less than 1.5 meters.
Trough distributors comprise a collection of interconnecting open troughs or channels having irrigation holes in the base to feed liquid to the packing below. At least one upper collection trough, or a simple pot on top of the lower troughs, feeds liquid to the lower troughs through a series of holes or overflowing notches. Vapor from the packing below passes upward between the liquid-containing troughs.
FIG. 1 shows a typical liquid distributor 10 of the trough type. Liquid from feed assembly 12 enters a pre-distributor 14, which distributes the liquid to the distributor. The distributor is mounted on a combined hold-down/support grate (not shown) above the packing (not shown).
After entering the distributor 10, the liquid flows in a plurality of channels or troughs 18 spaced apart by vapor risers 16 throughout the distributor. A typical main channel 17 and multiple troughs or channels 18 on each side of the main channel 17 are shown in FIG. 2. Liquid from the main channel enters each channel at the inlet end 20 of the channel and flows in a direction 22 away from the inlet end. Streams of liquid 24 then exit each channel through orifices or holes 26 in the bottom 28 of the channel. If the liquid does not flow from the holes in uniform directions, some areas of the packing below the distributor are under irrigated areas 30 while other areas of the packing are over irrigated areas 32, as shown in FIG. 2. Also, some of the liquid may impact internal structures between the bottom of the distributor and the packing, such as distributor supports/hold-down grates 34, as shown in FIG. 2. (These internal structures may support the distributor and/or hold down the packing.)
Some liquid distributors used in distillation processes are disclosed in U.S. Pat. No. 5,752,538 (Billingham, et al.); U.S. Pat. No. 5,240,652 (Taylor, et al.); U.S. Pat. No. 6,086,055 (Armstrong, et al.); U.S. Pat. No. 4,729,857 (Lee, et al.); U.S. Pat. No. 5,192,465 (Petrich, et al.); and U.S. Pat. No. 5,645,770 (McNulty, et al.).
The prior art distributors generally use three types of distribution regulation mechanisms: the weir type, where liquid flows horizontally through a gap; the orifice type, where liquid flows vertically, or horizontally, usually through a circular hole; and the pressure type, where feed under pressure is distributed through a series of spray nozzles. The orifice type is distinguished by the fact that the flow rate of liquid through the hole is proportional to the square root of the height of liquid above the orifice. For a narrow weir, the flow can be taken as being proportional to the height of liquid raised to the power 1.5. Use of orifices is often preferred because the effects of minor changes in the liquid level or the levelness of the distributor are reduced by having the flow rate being proportional to the height squared if a reasonable liquid depth is used. However, this comes at the expense of reducing the operating range of a simple distributor because the height available for the distributor is often limited. Weir type distribution is often preferred when a large amount of liquid must be distributed, high rangeability is required, or in the pre-distribution section of the distributor.
In the case of orifice type distribution, the thickness of the orifice material plays an important part in regulating the flow and the direction of the liquid stream. Orifices generally may be divided into two classesxe2x80x94those in thick material and those in thin material. For a material to be classified as thick, the liquid flow must be fully developed within the thickness of the material, which gives rise to a high L/D ratio (where L is the material thickness and D is the diameter of the hole). For a thin material, the L/D ratio is lower, normally below 1.0. For an orifice in a thick material, the stream generally will emerge in line with the axis of the orifice, while in a thin material the stream will emerge at an angle with the axis of the orifice, which angle is determined by the direction of any cross-flow velocity in the liquid above the orifice. In any distributor, this cross-flow velocity is caused by the natural movement of liquid to the distribution orifices.
Since the use of thin materials often is advantageous due to ease of bending and manufacturing, many liquid distributors are constructed using channels which are made from thin material and bent to shape. The orifices are punched or drilled through a thin metal sheet prior to the bending process of forming the trough shape. Unfortunately, an orifice in a thin material suffers from the characteristic that the cross-flow velocity near the entrance to the orifice will influence the direction of the exiting stream biasing it in the direction of said cross-flow velocity. Although experiments have shown that a relatively severe cross-flow velocity is required to significantly affect the actual flow rate of the stream, the fact that the stream does not leave in line with the axis of the orifice means that two problems are encountered: 1) the stream does not land where it is expected to on the packing; and 2) during quality testing of the distributor, it is difficult to measure the performance accurately. A stream that has an inaccurate trajectory (i.e., does not flow as desired to the packing) may come into contact with other components, such as distributor support/hold-down grates 34, as illustrated in FIG. 2.
The most common way of eliminating the problems associated with streams leaving the orifices in a non-vertical direction is to add some form of tube to the outlet side of the orifice. These tubes are normally part welded onto the underside of the channel, with the orifice located at the center of the tube. The tube then directs the liquid straight down, regardless of the actual trajectory that the liquid has when it leaves the orifice. However, the use of these tubes is both expensive, as each tube must be individually attached to the main channel and/or the troughs or channels, and cumbersome, as the tubes are vulnerable to damage during handling. Therefore, use of such tubes normally is limited to those areas of the liquid distributor where high cross-flow velocities are expected. An additional problem with the tubes is that the tubes correct the directional problem after the liquid has left the orifice. However, in extreme cases, when the flow rate through the orifice is high, the cross-flow velocities above the orifice can significantly affect the flow rate through the orifice, which will lead to non-uniform distribution of liquid onto the packing.
Another approach to addressing the problems of streams not flowing vertically from the distributor is disclosed in U.S. Pat. No. 5,051,214 (Chen, et al.), where a pre-distributor is extended over the troughs in order to transfer the liquid from the pre-distributor to the distributor over a wider area. By introducing liquid directly into the troughs, the cross-flow velocity is reduced at what would be the feed end of the trough. The primary shortcoming of this approach is the cost of the complex pre-distributor. Also, the design takes up some of the space at the top of the channels, thereby reducing the design height available for the liquid, thus reducing the operable range of the distributor.
It is desired to have an apparatus and a method for distributing a liquid in an exchange column with a liquid distributor which mitigates the effects of cross-flow velocity on the direction of liquid streams from the liquid distributor.
It is further desired to have an apparatus and a method for distributing a liquid in an exchange column with a liquid distributor which will reduce or prevent the bulk flow of liquid in directions that is not desirable.
It is still further desired to have an apparatus and a method for distributing a liquid in an exchange column with a liquid distributor which can better control the flow of liquid to specific areas of the liquid distributor.
It is still further desired to have an apparatus and a method for distributing a liquid in an exchange column having packing which eliminate or mitigate under irrigation and over irrigation of the packing.
It is still further desired to have an apparatus and a method which overcome the difficulties, problems, limitations, disadvantages, and deficiencies of the prior art to provide better and more advantageous results.
It is still further desired to have a method of assembling a liquid distributor for exchange columns which affords better liquid distribution than the prior art liquid distributors, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have a new, more efficient method for the distribution of a liquid and a vapor in exchange columns.
It is still further desired to have a liquid distributor that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It also is desired to have a more efficient air separation process utilizing a liquid distributor which is more efficient than the prior art.
The invention is an apparatus for distributing a liquid in an exchange column. The invention also includes a method for adjusting a flow direction of a stream of a liquid exiting an aperture in an elongated channel within a plate for distributing liquid in an exchange column. In addition, the invention includes a method for assembling a distributor for distributing a liquid to a packing in an exchange column.
A first embodiment of the apparatus includes a plate and at least one elongated internal baffle. The plate has at least one elongated channel, which has a first longitudinal axis, a bottom, and at least one aperture in the bottom. The internal baffle has a second longitudinal axis substantially parallel to the first longitudinal axis, and at least a substantial portion of the internal baffle is disposed in the channel.
There are many variations of the first embodiment of the apparatus. For example, the internal baffle may have a triangular shape or a zig-zag shape. In another variation, a part of the internal baffle is adjacent the aperture. In yet another variation, at least a portion of the internal baffle is perforated. In still yet another variation, the internal baffle has a plurality of edges, and at least one edge has a non-linear shape. In still yet another variation, the internal baffle has a plurality of perforations and divides the channel into generally parallel spaced apart first and second subchannels, the subchannels being in fluid communication across the perforations. In this variation, the first subchannel has at least one aperture and the second subchannel has a substantially fewer number of apertures (which can be zero) than the first subchannel.
A second embodiment of the apparatus is similar to the first embodiment, but includes a control baffle. At least a substantial portion of the control baffle is disposed in another channel having a third longitudinal axis at an angle with the first longitudinal axis and is in fluid communication with the channel having the first longitudinal axis.
Another aspect of the invention is an exchange column for exchanging heat and/or mass between a liquid and a vapor, the exchange column having at least one apparatus for distributing a liquid in the exchange column like the first embodiment of the apparatus discussed above.
Yet another aspect of the invention is a process for cryogenic air separation comprising contacting liquid and vapor counter-currently in at least one distillation column containing at least one mass transfer zone, wherein liquid-vapor contact is established by at least one packing, and wherein liquid is distributed to the packing by an apparatus like that in the first embodiment of the apparatus discussed above.
There are several steps in the first embodiment of the method for adjusting a flow direction of a stream of liquid exiting an aperture in an elongated channel within a plate for distributing liquid in an exchange column, the elongated channel having a first longitudinal axis, a bottom, and at least one aperture in the bottom. The first step is to provide at least one elongated internal baffle having a second longitudinal axis. The second step is to place at least a substantial portion of the internal baffle inside the channel in a position whereby the second longitudinal axis is substantially parallel to the first longitudinal axis.
There are several variations of the first embodiment of the method for adjusting a flow direction. For example, at least a section of the internal baffle may have a triangular shape or a zig-zag shape. In another variation, a part of the internal baffle is adjacent the aperture. In yet another variation, at least a portion of the internal baffle is perforated. In still yet another variation, the internal baffle has a plurality of edges, and at least one edge has a non-linear shape. In still yet another variation, the internal baffle has a plurality of perforations and divides the channel into generally parallel spaced apart first and second subchannels, the subchannels being in fluid communication across the perforations. In this variation, the first subchannel has at least one aperture and the second subchannel has a substantially fewer number of apertures (which can be zero) than the first subchannel.
In a second embodiment of the method for adjusting a flow direction of a stream of liquid exiting an aperture, there are several additional steps. The first additional step is to provide at least one control baffle. The second additional step is to place at least a substantial portion of the control baffle in another channel within the plate, the other channel having a third longitudinal axis at an angle with the first longitudinal axis and being in fluid communication with the channel having the first longitudinal axis.
A first embodiment of the method for assembling a distributor for distributing a liquid to a packing in an exchange column includes multiple steps. The first step is to provide the exchange column. The second step is to provide the distributor, which includes a plate and at least one elongated internal baffle. The plate has at least one elongated channel, the channel having a first longitudinal axis, a bottom, and at least one aperture in the bottom. The internal baffle has a second longitudinal axis substantially parallel to the first longitudinal axis, and at least a portion of the internal baffle is disposed in the channel. The third step is to install the distributor in the exchange column.
A second embodiment of the method for assembling a distributor is similar to the first embodiment, but the distributor includes an additional element. In this embodiment, the distributor includes at least one control baffle, and at least a substantial portion of the control baffle is disposed in another channel within the plate, the another channel having a third longitudinal axis at an angle to the first longitudinal axis of the channel.